For spin torque applications, the dry etching process of the magnetic tunnel junction (MTJ) is the critical step determining MTJ performance. The key issue is the sidewall damage caused by the MTJ etching process. A typical MTJ etched by a RIE (reactive ion etching) process is found to have sidewall damage, possibly caused by oxygen or other chemicals during the etching process and/or by exposure to oxygen after the etching process. The sidewall damage will lead to a size dependence of magnetic resistance ratio (DRR) which will reduce the read margin for the MRAM (magneto-resistive random-access memory) chip functionality. This damage will be getting worse as MRAM technology decreases in size to 65 nm technology and beyond.
This sidewall damaged layer around the MTJ is believed to be a chemically unstable layer. The thermal treatment of the semiconductor back end of line (BEOL) process will enhance the sidewall damage due to atom diffusion and will result in quickly degraded MTJ performance.
Several patents teach methods to remove sidewall damage. These include US Patent Applications 2017/0025603 (Hara), 2016/0020386 (Kim et al), and 2006/0132983 (Osugi et al). Other patents teach uses of chemical mechanical polishing (CMP), including U.S. Pat. No. 8,105,948 (Zhong et al) and U.S. Pat. No. 8,822,994 (Zhang et al). All of these references are different from the present disclosure.